The invention relates to an antitheft device for a motor vehicle and to a method for operating an antitheft device for a motor vehicle.
A known antitheft device for a motor vehicle (DE 196 12 452 C1) has a receiver which is arranged at the vehicle end and is designed to receive and demodulate signals in narrowband. If a portable code generator emits a code signal with code information, the receiver can receive and demodulate this code signal. The code information contained in the code signal is passed on to an evaluation unit in which this code information is compared with reference or setpoint code information. When the two correspond, an enable signal is generated which locks or unlocks doors or enables an immobilizer, and the internal combustion engine is started.
Such an antitheft device can also utilize harmonics of the fundamental component at the carrier frequency as the code information is also contained in the harmonics. These harmonics are found at integral multiples of the carrier oscillations. This ensures that, when there is narrowband interference on the transmission path, the code information can nevertheless be reliably received and evaluated. However, the harmonics have lower amplitudes than the fundamental components at the carrier frequency in the useful frequency band. As a result complex receivers are required.
However, this known antitheft device cannot prevent the transmission link being manipulated, that is to say cannot prevent illegitimate recording and reproduction of the recorded signals which would permit an unauthorized person to gain access to the motor vehicle.
A further known antitheft device (DE 199 57 536 A1) attempts to prevent such manipulation of the transmission link by measuring a distance between the motor vehicle and code transmitter by means of radar signals which are emitted in broadband and code signals which are additionally modulated and transmitted back. The emitted code signal is considered to be authorized only if this distance lies within a predefined range and if the code information corresponds to the setpoint code information. This antitheft device is however very complex and is based on a precise measurement of the distance between the code generator and the motor vehicle.